rtc: rtc-s3c: Add device tree support

Add device tree based discovery support for Samsung's rtc controller.

Cc: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>